1. Field of the Invention
This invention relates to an LCD (Liquid Crystal Display) drive circuit that generates segment signals and common signals to turn LCD segments on and off.
2. Description of the Related Art
In general, a segment type LCD device has a plurality of LCD segments and performs a display by applying a common signal and a segment signal to each of the LCD segments. The common signal has a waveform that is a repetition of a certain waveform pattern. The segment signal corresponding to display data is generated with reference to the common signal, and turns the LCD segment on or off. An LCD drive circuit to drive the LCD device as described above is described in Japanese Patent Application Publication No. H07-3 19418, for example.
Some of the LCD drive circuits operate in two driving states that are a ¼ duty driving state and a ⅓ duty driving state. The driving states of the LCD drive circuit have been set as described below.
Serial data including display data is provided with additional two bits of identification data (DD0, DD1) and inputted as four steps of divided serial data in the case of the ¼ duty driving state or as three steps of divided serial data in the case of the ⅓ duty driving state. In the ¼ duty driving state, the four steps of serial data each identified by each of the identification data (DD0, DD1)=(0, 0), (0, 1), (1, 0) and (1, 1) are inputted, and a control bit DT in the first step of the serial data, which corresponds to the identification data (DD0, DD1)=(0, 0), is set to “0”.
In the ⅓ duty driving state, on the other hand, the three steps of serial data each identified by each of the identification data (DD0, DD1)=(0, 0), (0, 1) and (1, 0) are inputted, and the control bit DT in the first step of the serial data, which corresponds to the identification data (DD0, DD1)=(0, 0), is set to “1”.
The LCD drive circuit incorporates a power-down reset circuit that outputs a reset signal to initialize the circuit in a certain range of power supply voltage which is lower than an operating voltage. A meaningless display immediately after power-on is prevented by doing so. The reset state continues after the power supply voltage reaches the operating voltage properly, and is held until one of the driving states is set. After the serial data is properly inputted and its identification data is recognized, the reset state is released when inputting of the four identification data is confirmed in the case where the control bit DT is “0” or when inputting of the three identification data is confirmed in the case where the control bit DT is “1”.
In the method to set the driving state described above, however, the driving state may be altered when a noise or the like causes an error in the control bit DT that solely determines the driving state. The altered driving state remains unchanged until the next first step of the serial data corresponding to the identification data (DD0, DD1)=(0, 0) is inputted properly. As a result, there is caused an abnormal display of the LCD segments. For example, all the four steps of serial data are inputted to operate in the ¼ duty driving state and the reset state is released. After that, if the control bit DT is mistakenly set to be “1” by a noise or the like when the first step of the serial data corresponding to the identification data (DD0, DD1)=(0, 0) is inputted, the driving state is altered to the ⅓ duty driving state and an unintended waveform of the signal is outputted.
Even when the control bit DT is not mistakenly changed, the conventional LCD drive circuit is structured so that the serial data corresponding to the other driving state can be inputted as long as its format is correct. For example, if the serial data corresponding to the ⅓ duty driving state is inputted while the LCD drive circuit is set to the ¼ duty driving state, there is caused a problem that an unintended display of the LCD segments is made based on the serial data.